The present invention relates generally to a non-dispersive infrared analyzing apparatus for measuring the concentration of a gas in a gas mixture, and more particularly to one utilizing a modulated infrared radiation beam.
The use of a modulated infrared beam to analyse a gas mixture by means of a double-celled test chamber is already well known in the prior art. Such arrangements make use of a double-layered absorption or detection chamber, in which the modulated beam traverses sequentially first a forward portion or layer, and then a rear portion or layer. The path length of the radiation beam in the forward portion is shorter than the path length in the rear portion.
In such gas analyzers, the predominant energy absorbed is associated with the most probable wavelength ranges, for example in the center of the absorption line spectrum. In the rear portion or layer, the less probable side spectrum line regions would be absorbed. This is possible since the forward layer has already absorbed the radiation corresponding to the easily absorbable wavelength regions. The difference in the absorption of beam energy of the radiation corresponding to the two cells of the test chamber is a measure of the concentration of the analyzed gas in the gas mixture.
The null point of the measurement, corresponding to a zero concentration of the analyzed gas in the gas mixture, must result in a zero output signal. This therefore implies that the measurement of the null point of the pressure impulse based on the absorption of the modulated beams in both of the provided layers, must be equalized both in terms of amplitude and phase, and the membrane condensor adjusted to a dynamical operating balance.
As a result of the absorption of the modulated beam in the absorption chamber, a pressure impulse is created depending on the total volume and the absorbed power level. The time lag of the pressure impulses and therefore the phase lag will essentially be determined by the heating and cooling effect depending on the volume as well as the heat conductivity of the measuring chamber for the particular gas or gases under consideration. The equalizing of the impulse amplitudes is achieved by selecting the length of the absorption chamber as well as the partial pressure, in addition to providing a non-cylindrical geometry for the rear portion. But because of the different geometry of the chamber, different capillary diameters, as well as the differences in the characteristics of the gas in both layers, there are different time constants for heating and cooling; the equalization of the pressure impulses in such prior known devices is therefore not perfect. The inequality originates from a phase difference between the signal from the forward and the rear layer of the absorption cell, even producing a resulting difference signal from full amplitude equalization which is larger than the difference of respective end range values for the signal being measured.
Even with the most close fitting parts and uniform production and packing techniques for the measurement chamber, absolute symmetry is not achieved, since the symmetry is effected by the beam geometry and the wavelength-dependent beam intensity, for example, the emitter temperature.
It is desirable to obtain an exact equalization both in terms of the amplitudes and the phases of the pressure impulses between both forward and rear portions of the absorption chamber. It is particularly advantageous to achieve this equalization for the amplitude and phase in a simple and practical manner. There have been attempts to provide full equalization through interfering with the beam geometry by means of an adjustable shield before the absorption chamber. However, in such a situation the absorbed energy in both of the layers would vary and at the same time there would be an undesirable variation in the range of the sample or measured signal.
Furthermore, the placing of a shield between the absorption layers or cells does not result in any satisfactory solution to the problem affecting the amplitudes of pressure impulses, though the different phase differences corresponding to different cooling and heating times would scarcely vary. Another method is to attempt an equalization by means of a pneumatic shunt or a storage buffer. Such an arrangement is, however, expensive and inconvenient and a loss of sensitivity is associated with the increased volume.
The same process is also applicable to the method of equalizing the pressure impulses of each absorption layer by means of a passive equalization volume, which is matched with the volume of the other absorption layer by means of a connecting condensor membrane. In this case there is a diminution of the signal by a factor V.sub.1 /(V.sub.1 +V.sub. passive volume).
It has also been proposed to connect both absorption layers by means of an axially moveable window operable from outside the measuring chamber, or utilizing between both absorption layers a compensation layer with an adjustable gas pressure. With the help of such an arrangement, it is possible to equalize the amplitude of the signals while the geometry of the layers, and therefore the heating and cooling times, remain unchanged, and the phase equalization is not achieved. Moreover, it follows from the first solution through the variation of the volume of both chambers a variation in the measurement range results.
Another method is to compensate for the different amplitude and phases by means of a difference signal which utilizes an adjustable amplitude and phase factor to thereby adjust the parameters to the null point measurement. This is, however, a relatively complicated method.
It is also known to provide a metal cone in the interior of the rear absorption cell or layer, and adjust its axial position. With this method only the rear absorption layer will be influenced, so that no signal diminution or sensitivity variation would arise. However, the variation of the position of the cone will influence the amplitude or magnitude of the pressure impulses, though the influence on the phase will likewise be small.